A tire comprises two beads which provide the mechanical connection between the tire and the rim on which it is mounted, the beads being respectively connected by two sidewalls to a tread which is intended to come into contact with the ground via a tread surface.
In what follows, the circumferential, axial and radial directions respectively denote a direction tangential to the tread surface in the direction of rotation of the tire, a direction parallel to the axis of rotation of the tire, and a direction perpendicular to the axis of rotation of the tire. “Radially on the inside” and “radially on the outside” respectively mean “closer to” and “further away from” the axis of rotation of the tire. “Axially on the inside” and “axially on the outside” respectively mean “closer to” and “further away from” the equatorial plane of the tire, the equatorial plane of the tire being the plane passing through the middle of the tread surface of the tire and perpendicular to the axis of rotation of the tire.
A radial tire more particularly comprises a reinforcement comprising a crown reinforcement radially on the inside of the tread and a carcass reinforcement radially on the inside of the crown reinforcement.
The carcass reinforcement of a radial tire for a heavy vehicle of the construction plant type usually comprises at least one carcass layer consisting of reinforcers, generally made of metal, coated in an elastomeric coating material or coating compound. In the field of tires, an elastomeric material usually obtained by blending components of the material is usually referred to as a compound. The carcass layer comprises a main part connecting the two beads together and wrapped, within each bead, from the inside towards the outside of the tire, around a bead wire to form a turnup. The metal reinforcers are substantially parallel to one another and make with the circumferential direction an angle of between 85° and 95°, in the case of the main part, and an angle of between 75° and 105°, in the case of the turnup.
The bead wire is made up of a circumferential reinforcing element, generally made of metal, surrounded with at least a coating material, nonexhaustively made of elastomer or textile. In what follows, the diameter of the bead wire is the name given to the diameter of the substantially circular meridian section of the bead wire which is the diameter of the circle circumscribed around the meridian section of the bead wire, which consists of the metal circumferential reinforcing element surrounded by its coating element. The portion of bead wire in contact with the carcass reinforcement contributes to reacting tensile forces in the carcass reinforcement upon inflation, by coupling with the carcass reinforcement. This contribution to the reaction of tensile forces is dependent on the torsional stiffness of the bead wire and on the length of the turnup. In the usual scenario in which the bead wire has high torsional stiffness, the tensile forces upon inflation are essentially reacted via the bead wire, with the turnup making a secondary contribution.
The turnup in each bead allows the carcass layer to be anchored to the bead wire in that bead. In the case of a tire for a heavy vehicle of the construction plant type, the turnup is generally long, i.e. its free end is radially closer to the axially outermost point of the carcass reinforcement in the sidewall of the tire than to the axially outermost point of the bead wire.
Each bead also comprises a filling element extending the bead wire radially outwards and of substantially triangular shape. The filling element is made of at least one filling elastomeric material or filling compound and often made up of a stack, in the radial direction, of at least two filling compounds with different chemical compositions. Furthermore, the filling element axially separates the main part from the turnup.
A compound, after curing, is characterized mechanically by tensile stress-strain characteristics which are determined by tensile testing. This tensile testing is carried out by a person skilled in the art, on a test specimen, using a known method, for example in accordance with international standard ISO 37, and under normal temperature (23+ or −2° C.) and relative humidity (50+ or −5% rh) conditions defined by international standard ISO 471. The elastic modulus at 10% elongation for a compound, and expressed in mega pascals (MPa), is the name given to the tensile stress measured for a 10% elongation of the test specimen.
A compound, after curing, is also characterized mechanically by its hardness. The hardness is notably defined by the Shore A hardness determined in accordance with standard ASTM D 2240-86.
When the vehicle is driving along, the tire, mounted on its rim, inflated and compressed under the load of the vehicle, is subjected to bending cycles, particularly at its bead and at its sidewalls.
The bending cycles lead in particular to stresses and strains, mainly in shear and in compression, in the filling compounds, because of the flexing of the bead on the rim flange.
Document EP 2216189 describes a tire bead the endurance of which is improved by reducing the compressive strains in the turnup as the bead flexes on the rim in use. This objective is achieved by a turnup which is such that the distance between the turnup and the main part decreases continuously, radially towards the outside, from the bead wire as far as a minimum distance and then increases continuously as far as a maximum distance. The turnup extends radially on the outside of the point of the turnup that corresponds to the maximum distance between the turnup and the main part.
Document JP 2010274862 also describes a tire bead the endurance of which is improved, as the bead flexes on the rim in use, in the case of a bead, as described by document EP 2216189, with a turnup that is such that the distance between the turnup and the main part decreases continuously, radially towards the outside, from the bead wire as far as a minimum distance, then increases continuously as far as a maximum distance. This objective is achieved by virtue of the presence of a filling element between the main part and the turnup, comprising a hard first compound extending radially towards the outside from the bead wire, and a second filling compound extending radially towards the outside from the hard first compound. The second filling compound is present at least in part in the region in which the distance between the main part and the turnup is minimum. This design makes it possible to decrease shear forces in this region and therefore improve the endurance of the bead further.
In the technical solutions described respectively by the aforementioned documents EP 2216189 and JP 2010274862, a turnup such that the distance between the turnup and the main part decreases continuously, radially towards the outside, from the bead wire as far as a minimum distance, causes the turnup to be brought significantly closer to the main part of the carcass layer.
This meridian profile of turnup makes it possible to reduce the compression in the turnup, or even place it under tension, because the assembly consisting of the turnup, the filling element and the main part of the carcass layer behaves mechanically like a beam the cross section of which decreases radially towards the outside.
Moreover, bringing the turnup closer to the main part leads to a corresponding thickening of the portion of bead axially on the outside of the turnup and consisting of at least one compound. This thickening leads to a reduction in shear loadings at the interface between the axially outer compound adjacent to the turnup and the turnup, and therefore leads to better fatigue strength of this interface, this contributing to improving the endurance of the tire.
On the other hand, bringing the turnup closer to the main part leads to a reduction in thickness of the filling element interposed axially between the turnup and the main part of the carcass layer. This reduction in thickness leads to an increase in shear loadings at the interface between the filling element and the main part of the carcass layer, and therefore to lower fatigue strength of this interface, this contributing to impair the endurance of the tire.